By David Gibbons
Revised April, 2007
These days many home theater speaker systems feature very small speakers for the front, center, and surround speakers. This is particularly true of the "home theater in a box" systems.
This has the advantage of allowing you to fit a home theater into a closet, if you should happen to use one as your viewing room. Such small speakers are also very unobtrusive, and so therefore may be more acceptable to other family members. However, if you check carefully into the specifications for what is actually tucked inside those small speaker enclosures, you may find only one small (like 2 inch!) speaker.
That one small speaker is being asked to cover all the frequencies from where the speaker system's "subwoofer" stops on up to the top frequency delivered by the system. Due to their small physical size, the speakers will not do too good a job of delivering real power at mid-range frequencies. The manufacturer may design the little speakers with "long throw" capability, which will help them move a little more air for their size. Still, the best analogy I can think of to describe what is happening when using small speakers in mid-range frequencies is comparing a big powerful 8 cylinder car doing 90 MPH to a little four-cylinder economy car trying to go the same speed. The automobile with the big engine will be cruising along, while the little car will be thrashing as hard as it can just to keep up.
To be fair, headphones can produce excellent sound with plenty of volume using single speakers about the size of the small speakers we are talking about. What's the difference? Consider this: how many cubic inches of air are there in that headphone cup over your ear? 1 to 3 cubic inches, probably. How many cubic inches in a 10' by 12' room with a 8' ceiling? 1,658,880...a LOT more air to fill with sound, and a lot more distance between your ears and the speakers.
In addition, true "tweeter" speakers are designed to perform well at the highest audible frequencies, and the single speakers cannot work as well at the top frequencies either. (If you are older, and have lost your higher frequency hearing, this may not be so much of a big deal.)
The home theater speaker set manufacturer may try to make up for the lack of low end in the little speakers by designing the so-called subwoofer to work at frequencies up to 120 Hz (Hz=Hertz, a unit of frequency) or 150Hz (or even higher!). If you check the frequency range of such "subwoofers", you may find that they end up only reaching down to 40 or 50 Hz, which doesn't make for much deep bass. ( Which is what subwoofers are supposed to produce. )
Further, another difficulty arises from allowing the "subwoofer" to handle frequencies above 80 Hz. As you move upwards in frequency from 80 Hz, it becomes easier to tell where a sound is coming from. Below 80 Hz, it is much harder to tell where a sound comes from. This allows a sub-woofer that doesn't produce frequencies above 80 Hz to be located where it is convenient, or where it makes for the best bass. This is why Lucasfilms' THX standard uses 80 Hz as the crossover point between the subwoofer and the rest of the speakers in the system.
If your system uses a crossover frequency much higher than 80 Hz, make every effort to put the "subwoofer" right near the television, so the illusion that the sound is coming from the screen is maintained even at the higher subwoofer frequencies.
If the home theater speaker set manufacturer is careless, or cares more about delivering deep bass, the subwoofer supplied may not reach "up to" the frequency where the little speakers start to be able to produce a useful signal. The result will be a "hole" between the top end of the subwoofer frequency range and the bottom end of the little speaker's frequency range. Low male voices will sound thin, and music with low-midrange frequencies will also be weak. Most likely the hole will lie between 100 and 150Hz.
Since our hearing is so adaptable, folks can listen to home theater speaker systems with small speakers, and not know what they are missing. Before you purchase a home theater system using small speakers, listen to the same movie or music on a good home theater speaker set where each of the center and front speakers have at least a 4 in. diameter mid-range speaker in addition to a tweeter, and compare that sound (at the same volume level) to what you hear on the small speakers.
This way you will be aware of all the trade-offs in using small speakers.
